Available quantum cryptography systems are referenced in Quantum Opt. -3 201 (1991), by S. F. Seward et al., Phys. Rev. Lett. 68 3121 (1992), by Ch. Bennet, Electronics Letters 30 1875 (1994), by P. D. Townsend et al., and Electronics Letters 31 232 (1995), by I. D. Franson et al.
Quantum cryptography is a method for a secure transmission of random keys, used to encrypt messages, over publicly accessible optical communication channels. Attempts to intercept the message disturb key generation by changing the exchanged quantum states of the light, with the result that the information encrypted by the key cannot be used. Attempts to intercept the message can be detected.
A message can be transmitted securely only if it generally cannot be intercepted or comprehended, even by the network operator himself. For this purpose, a random key of the same length as the message itself must be reliably exchanged between the transmitter and receiver. Quantum optical methods, which are described briefly below, have been suggested for this purpose.
The various physical properties of the quantum state of light cannot be measured precisely at the same time. This is felt the most strongly with polarization. Because the latter is composed of two independent directions of polarization, and the polarization analysis involves detecting a photon behind an analyzer (and thus destroying it), a single measurement can identify only one component of the polarization state at a time, while the others remain unidentified.
An eavesdropper must measure the photon he has intercepted and then replace it so that its absence is not detected. However, he cannot precisely identify the polarization state and therefore cannot obtain any precise information and also has no way to precisely duplicate the photon. The secure distribution of a statistical key between a transmitter and a receiver takes advantage of this fact. The transmitter and receiver can precisely receive the message since they subsequently exchange additional information about the base system in which the polarization is measured. This information is of no use to the eavesdropper.
A feature of conventional quantum cryptography methods is the ability to switch a polarizer at the transmitter""s location and switch an analyzer at the receiver""s location. Electrooptical modulators which either contain an electrooptical switch that transmits the light to individual, permanently set polarizers or analyzers on preset channels, as suggested in Mod. Optics 41 2405, by J. Breguet et al., or which are designed as switchable retarding elements as suggested in Mod. Optics 41 2425 (1994) written by P. Townsend et al. are used for this purpose.
FIG. 1 shows a principle of a conventional quantum cryptography device according to the polarization setting method. The transmitter has a single-photon source 2, such as a reduced-power laser, while the receiver has a detector for detecting individual photons 6, and both have a polarization setter, such as a linearly polarizing analyzer which can be rotated into positions x, xxe2x80x2, y or yxe2x80x2 (e.g., a polarizer 3, an analyzer 5, with a glass fiber 4 connecting).
The transmitter and receiver are connected by a quantum channel 1 in which individual photons are transmitted without any active amplification, as well as by a conventional line, such as a telephone or radio connection. Two random number generators independently adjust the analyzers at the transmitting and receiving ends each time an individual quantum state (photon) is transmitted.
An interferbmetric phase measurement for distributing the keys according to quantum cryptographic means can also be used instead of the polarization measurement, as suggested in Mod. Optics 41 2425 (1994), by P. Townsend et al. To do this, a defined optical path must be inserted into two interferometers at the transmitting and receiving ends. This can be done by adjusting a mirror or by electrooptical retardation in suitable materials, such as lithium niobate.
In available quantum cryptography systems according to the polarization setting method, which actually permit high speeds via the transmission channels, there are long switching times of the optical modulators used for this purpose.
An object of the present invention is to provide a faster quantum cryptography system according to the polarization setting method.
A quantum cryptography system according to the present invention uses electrooptical liquid crystal modulators which change the polarization and which are designed as electrically rotatable retardation plates whose two birefringent axes are rotated by the applied electrical field around an angle xcex8 which depends on the strength of this field. Thus, a xcex/2 retardation plate rotates the elliptically polarized light striking the plate around an angle 2xcex8 at speeds in the microsecond range. The quantum cryptography system can be used for interception-proof data transmission over transmission links that are accessible to the public.